1. Field of the Invention
The present invention relates to an irreversible circuit device such as a circulator and an isolator adapted to a duplexer.
2. Description of the Related Art
The configuration of a traditional irreversible circuit device will be described with reference to FIGS. 6 and 7. The traditional irreversible circuit device is configured of a U-shaped first yoke 51, a magnet 52 disposed inside the first yoke 51, a disk-shaped ferrite member 53 disposed under the magnet 52, three central conductors 54, 55 and 56 made of metal plates mounted on the ferrite member 53 at an angle of 120 degrees spaced as parts of them cross each other, a holding member 57 for holding the ferrite member 53, and a U-shaped second yoke 58.
In addition, the ferrite member 53 having the central conductors 54, 55 and 56 mounted thereon is inserted into a hole 57a of the holding member 57, and is covered with the first and second yokes 51 and 58 from above and below in a state that the magnet 52 is disposed on the ferrite member 53.
Then, the first and second yokes 51 and 58, in which the U-shaped parts are alternately put together, are combined to form a magnetic closed circuit by the first and second yokes 51 and 58 in a state that the magnet 52, the ferrite member 53 and the holding member 57 are sandwiched by the first and second yokes 51 and 58.
Furthermore, as shown in FIG. 7, the three central conductors 54, 55 and 56 are formed of thin metal plates, and each of them is connected to a capacitor C, which are configured of input or output first conductor parts 54a, 55a and 56a and second conductor part 54b, 55b and 56b on the ground side connected to the first conductor parts 54a, 55a and 56a. In each of three central conductors 54, 55 and 56, the first conductor parts 54a, 55a and 56a and the second conductor parts 54b, 55b and 56b are placed on the ferrite member 53.
Then, for example, when a signal is inputted to the first conductor part 54a of the central conductor 54, a current I flows through the second conductor part 54b. Subsequently, a magnetic flux is generated on the second conductor part 54b side, and the magnetic flux affects the second conductor part 55b through the ferrite member 53 placed between the second conductor part 54b and the second conductor part 55b as a magnetic path G.
Consequently, the current I is generated in the second conductor part 55b of the central conductor 55 and is outputted from the first conductor part 55a. 
Moreover, when the magnetic flux passes through the ferrite member 53 placed between the second conductor part 54b and the second conductor part 55b as the magnetic path G, unnecessary current is generated in the first conductor part 56a of the central conductor 56 placed therebetween, and the energy to be transmitted is lost in a large amount in the process of transmission. Consequently, the insertion loss is increased, and the isolation is deteriorated.
Additionally, also in the cases where a signal is inputted from the central conductor 55 and outputted from the central conductor 56 and where a signal is inputted from the central conductor 56 and outputted from the central conductor 54, the insertion loss is increased and the isolation is deteriorated by the same principles as described above.